The present invention relates to a process for manufacturing sintered bodies by molding powders of metals, ceramics or the like utilizing a plastic molding process. More particularly, it relates to the composition of plastic molded parts such as injection molded parts and to a process for debinding the plastic molded parts.
Heretofore, there has been known a process for manufacturing sintered bodies, in which metal powders or ceramics powders are blended with an organic binder, and the mixture is kneaded to impart the powders with sufficient flowability, then the mixture is molded by plastic molding such as a injection molding process to form a molded part or compact, and the compact is debinded(debind: removing binder from a molded part) and sintered. The term "plastic molding" as used herein refers to injection molding, transfer molding and hot extrusion molding. In this case, examples of the debinding process include (a) a process for heating the plastic molded part to decompose and vaporize the organic binder, and (b) a process for extracting the organic binder from the plastic molded part using a solvent.
The process (a) has a defect that it takes a very long time for the molded part to be debinded without causing its deformation. More particularly, plastic molding processes generally make use of the phenomenon that the kneaded mixture composed of the powder and the organic binder softens and become flowable when it is heated, and one can naturally expect that the molded part softens and deforms upon heating. It might be a solution to prevent this defect to remove a portion of the organic binder contained in the kneaded mixture before the temperature reaches the temperature at which the molded part begins to undergo deformation (hereafter, sometimes referred to as "deformation temperature"). In this case, however, it takes a long time to obtain desired effects if the organic binder is removed by decomposing or vaporizing it at temperatures below its deformation temperature. On the contrary, if the organic binder decomposes or vaporizes readily at temperature below its deformation temperature, the flowability of the kneaded mixture upon the plastic molding is unstable and in addition the recycle of the runners and sprues after the plastic molding is impracticable.
Another process in which the deformation of the molded part is prevented by controlling the degree of its softening at the time of debinding results in that the flowability of the kneaded mixture upon plastic molding is deteriorated, which makes molding with high precision difficult. On the other hand, processes in which thermosetting resins are used as the organic binder or sublimable substances are added in order to prevent the deformation of the molded part upon debinding cause problems that the flowability of the kneaded mixture is deteriorated and that the recycle of the runners and sprues is impossible. Further problem involved in the process (a) above is that swelling or cracks occurs in the plastic molded part due to abrupt decomposition or vaporization of the organic binder in the initial stage of debinding when the rate of the debinding is raised.
Accordingly, when a portion of the organic binder is removed by extracting it with a solvent at temperature below the deformation temperature of the molded part according to the process (b) above in order to solve this problem, and the space formed in the molded part by the removal of the organic binder serves as a guide passage for decomposition gases, which enables the gases generated by the decomposition or vaporization of the remainder of the organic binder to be readily discharged, thus avoiding the occurrence of deformation upon subsequent heating resulting in that swelling or cracks of the plastic molded part hardly occurs. As in the case of (a), recycle of the runners and sprues is possible since there is no need to employ thermally unstable components.
However, in the process (b) above, in order to extract those organic binders which have been generally used in powder plastic molding processes, it has heretofore been necessary to use organic solvents such as the hydrocarbon or the halogenated hydrocarbon which are in most case harmful to humans. For this reason, a special installment is needed in order not to discharge vapor of the organic solvent and the like out of the system and to prevent humans from being brought in direct contact with the organic solvent. Furthermore, the manufacuture cost increases since organic solvents are generally expensive. Also, full care must be taken for the administration of the organic solvents since there is a danger that they readily catch fire if they are carelessly or wrongly treated with.
Also, a process in which debinding by heating is conducted after extracting, among the organic binder, only plasticizers and lubricants might be usable. In this case, however, it is impossible to increase the amount of the plasticizer or lubricant to be added to the kneaded mixture more than ever since the strength of the molded part is deteriorated or poor molding such as bleeding occurs upon the plastic molding with increased amount of the plasticizer or lubricant. Therefore, this process has been felt unsatisfactory in order to prevent the deformation or swelling of the molded part at the time of debinding by heating.